The overall objective of this proposal is to examine the physiological role of lactoferrin in mammalian systems. Lactoferrin (LF) is an iron-binding glycoprotein found mainly in human colostrum and other exocrine secretions. The protein is structurally related to serum transferrin (TF) which provides the major physiological route for delivery of ferric iron to cells. Iron delivery by transferrin is dependent upon the intracellular release of iron from TF at acidic pH. Lactoferrin differs from its serum counterpart in its ability to retain iron at very low pH (2.2). This property has led to speculation regarding a physiological role for LF in delivery of iron to cells or sequestration of toxic iron from cells. Several nutritional functions have been ascribed to lactoferrin. The high bioavailability of iron from human breast milk, together with the identification of specific receptors for lactoferrin (but not for transferrin) on intestinal brush border membranes supports a role for LF in regulation of Fe absorption by the intestine. It has also been suggested that the protein may promote growth of the gastrointestinal tract in neonates. Finally, the extremely tight binding capacity of LF for iron (Kd=10-36M) renders the protein a natural antibacterial agent. Lactoferrin deprives bacteria of essential iron required for growth and in this capacity may contribute to resistance against infections caused by E. coli. These properties suggest that LF may play a role in protection against iron deficiency anemia, microbial diarrhea and maintenance of the G.I. tract. However, definitive proof for such a role in in vivo is not yet available. In this proposal, we intend to provide a direct approach to examine nutritional requirements for lactoferrin in vivo. We intend to examine the physiological effects of LF deprivation in vivo by generating a transgenic mouse model incapable of expressing endogenous lactoferrin; and furthermore to examine the effects of supplementation with exogenously added recombinant human lactoferrin.